Download SoftRock RXTX Ensemble USB Interface

SoftRock RXTX Ensemble
USB Interface
DD+
Reference 1
Purpose and Function
The USB interface provides a way for the PC, running SDR (Software Defined Radio)
software to control the local oscillator and PTT functions.
The transceiver is controlled by an Attiny85 microcontroller, programmed as a USB device to provide PTT
control, paddles input, and local oscillator frequency control, in a galvanically isolated area of the PCB. 1
Theory and Design
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This is a really neat design for USB using only a small microprocessor and a
handful of parts
Speed is USB 1.0 at 1.5 Mbits/sec which would seem to slow but it is only used
for control, the Rx and Tx data is not passed through the USB bus but instead is
passed as analog stereo to and from the PC sound card. Only Local Oscillator
frequency selection, CW1, CW2 and PTT control signals, are passed across the
USB interface.
The assembly manual provides credit and a link to PE0FKO for the firmware and
links to his website at http://home.ict.nl/~fredkrom/pe0fko/SR-V9Si570/index.shtml#install
That web page states the firmware is based on the free USB AVR CODE by
DG8SAQand links to http://www.obdev.at/products/vusb/index.html.
Under Links this web page refers to http://www.xs4all.nl/~dicks/avr/usbtiny/
where the following description of this circuitry is found.
USBtiny is a software implementation of the USB low-speed protocol for the Atmel ATtiny
microcontrollers. Of course, it will also work on the ATmega series. The software is written for an
William R. Robinson Jr.
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2011 All rights Reserved
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SoftRock RXTX Ensemble
USB Interface
AVR clocked at 12 MHz. At this frequency, each bit on the USB bus takes 8 clock cycles, and
with a lot of trickery, it is possible to decode and encode the USB waveforms by software. The
USB driver needs approximately 1250 to 1350 bytes of flash space (excluding the optional
identification strings), depending on the configuration and compiler version, and 46 bytes RAM
(excluding stack space). The C interface consists of 3 to 5 functions, depending on the
configuration.
USB uses two differential data signals, D+ and D-, which are normally complementary. However,
the end of a packet is signalled by pulling both signals low. Data is not transmitted directly on the
USB bus, it is NRZI encoded first. This means that a "0" bit is encoded as a bit change, and a "1"
bit is encoded as no bit change. After 6 "1" bits, "bit stuffing" takes place to force a change on the
USB signal lines.
The software is interrupt driven: the start of a USB packet triggers an interrupt. The interrupt
handler synchronizes with the sync byte, removes the NRZI encoding and bit stuffing, and stores
the packed in one of the two RAM buffers. Two buffers are used so that the next packet can be
received while the current one is being processed. Depending on the packet type, a reply packed
may be sent back immediately in the interrupt handler. 2
USB CONTROL
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The 3.3V zener diodes D1 and D2 are used to reduce the 5 volt outputs from
the ATTiny85 to 3.3 volts. (This complies with the USB requirement of a
high being between 2.8 and 3.6 Volts. 3
The 68 ohm resistors, R1 and R2 limit the current into the ATTiny85
o (5-3.3V)/68 ohms = 25 mA.
o The ATTiny85 has a maximum of 40 mA for each I/O with a 60 mA
total. 4(p166-7)
R5 is used to pull up D- to signal to the host PC that the device that this is a
low speed device. Normally this is done with a 1.5K resistor to 3.3V. 3
However in this case it is pulled up to 5V
o Current with 3.3V across a 1.5Kohm resistor
 I = 3.3V/1.5K = 2.2 mA
o Resistor for same current pulled up to 5V
 R= 5V/ 2.2mA = 2.27K
 R5 and R2 both limit the current and add up to 2.268 ohms well
within 5% resistor tolerance
R6 provides a weak Pullup on D+ , perhaps it for when the USB is not pulled
in. I am not sure if this part is really needed
o I detected no failures using CFGSR software with R6 removed.
C50 is just a bulk capacitor for the USB 5V power supply to the ATTiny85
I2C
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ATTiny85 PB1 is used as a bidirectional I/O for the I2C SDA line.
ATTiny85 PB3 is used as a bidirectional I/O for the I2C SDC line.
R10 and R9 provide the required pull up resistors required by the I2C
protocol. 5 The ATTiny85 I/O are tri-stateable so can emulate the required
Open-drain behavior. 4(p55)
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2011 All rights Reserved
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SoftRock RXTX Ensemble
USB Interface
CW1 INPUT
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CW1 comes from J2-5 and goes to U1-1 (PB5). The input is active low.
R3 provides a pull up resistor so the input does not float when the key/paddles
are not connected or open.
R8 limits the current into/out of the ATTiny85 below the required
o When the J2-5 is floating the current from the 5V supply and into the
ATTiny85 is
o I=5V/(4.7K+4.7K) or ~ 0.5 mA.
o When the J2-5 is closed to ground the current from the 5V supply is
o I=5V/(4.7K) or ~ 1.0 mA.
CW2 INPUT/SDA out
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THE ATTiny85 PB1 Is used for two purposes, It is used as an output (along
with PB3) to control the Local Oscillator Frequency, and it is use as an input
for CW2 from J2-2
This is compatible because conceptually the user would not want to be
transmitting while adjusting the output frequency.
Both I2C and the paddles use a pull up resistor for High values so it is not
obvious to me why R4, R7 and Q1 are required.
PTT ISOLATION
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U4 provides opto-isolation from the USB power and the rest of the card
R11 limits the current to the LED in U4. Th opto-isolator has a Forward
voltage of 1.2V minimum and a max forward current of 20mA. 6
o R11min = (5-1.2)V/20mA = 190 ohms
o So 470 ohms is well within the requirements
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2011 All rights Reserved
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SoftRock RXTX Ensemble
USB Interface
Calculated
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NONE
Simulation
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NONE
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2011 All rights Reserved
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SoftRock RXTX Ensemble
USB Interface
Real Circuit
USB CONTROL
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The Trace below shows the zener diodes keeping the D- output back to the PC
at 3.3 Volts.
The pulses on the left side are from the ATTiny to the PC.
o D- is at the zenor voltage of 3.3Volts
o PB2 is about 180 mV higher than D- due to the voltage drop across the 68
ohm R2
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The pulses on the right are from the PC to the ATTiny85
o D- is higher than the zenor voltage of 3.3Volts suggesting that the PC has
significant drive power
o PB2 is about 110 mV higher than D- due to the voltage drop across the 68
ohm R2
o The voltage drop across the 68 ohm resistor is less as little current goes
into PB2 when it is an input then comes out when it is an output what
little current that does flow is more likely through the 2.2K R5
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How zenors and 68 ohm resistors do voltage conversion
o Magenta = Do Green = PB2
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The two traces below show the changes when connected to the PC directly
verses using a USB hub. Changing from PC USB port to USB HUB changes
voltages on the right side only suggesting the right side is from the PC to the
ATTiny
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SoftRock RXTX Ensemble
USB Interface

With HUB
o Top D+
o BOT D-
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With PC
o Top D+
o BOT D-
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SoftRock RXTX Ensemble
USB Interface
USB PAYLOADS
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A USB traffic analyzer was used to study the traffic across the USB interface
between the PC and the ATTiny85.
o A screen shoot from the tool is shown below
o The table below shows some of the frequency vales that were requested
using CFGSR software and the values sent across the interface
Requested
Frequency
Packet
Payload
2,000,000
1000000
16777216
8.388608000000
2,000,001
1000008
16777224
8.388607805696
2,000,002
1000011
16777233
8.388608111392
2,000,003
1000019
16777241
8.388607917088
2,000,004
1000022
16777250
8.388608222784
2,000,005
100002A
16777258
8.388608028480
2,000,006
1000032
16777266
8.388607834177
2,000,007
100003b
16777275
8.388608139872
2,000,008
1000043
16777283
8.388607945568
2,000,009
100004b
16777291
8.388607751265
2,000,010
1000054
16777300
8.388608056960
2,000,020
10000A8
16777384
8.388608113919
2,000,030
10000FC
16777468
8.388608170877
2,000,040
1000150
16777552
8.388608227835
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Dec(packet)
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2011 All rights Reserved
Packet value/
Requesed Frequency
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SoftRock RXTX Ensemble
USB Interface
2,000,050
10001a3
16777635
8.388607784805
2,000,060
10001F7
16777719
8.388607841765
2,000,070
100024B
16777803
8.388607898724
2,000,080
100029F
16777887
8.388607955682
2,000,090
10002F3
16777971
8.388608012639
2,000,100
1000347
16778055
8.388608069597
2,000,110
100039B
16778139
8.388608126553
30,000,000
0F000000
251658240
31,000,000
0F800000
260046848
8.388608000000
32,000,000
10000000
268435456
8.388608000000
33,000,000
10800000
276824064
8.388608000000
34,000,000
11000000
285212672
8.388608000000
35,000,000
11800000
293601280
8.388608000000
36,000,000
12000000
301989888
8.388608000000
37,000,000
12800000
310378496
8.388608000000
38,000,000
13000000
318767104
8.388608000000
39,000,000
13800000
327155712
8.388608000000
40,000,000
14000000
335544320
8.388608000000
o It appears that the value sent is the requested frequency * 8.3886080
o I also noted that 2 raised to the 23 (0x800000) is equal
to 8388608 decimal.
o I do not know of any reason for this ratio other than to speculate it is
useful for the firmware inside the ATTiny85
o I posted this question to the softrock40 Interest group have not received
any replies.
 http://groups.yahoo.com/group/softrock40/
 # 52262 USB Data Packets Format Jan 28, 2011 6:36 pm
I2C
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Below is a logic trace of the ATTiny85 accessing the Si570 (Write to address
55) Note that clock stretching is very evident. The byte takes about 145 usec
for a rate of about 55 Kbits/sec.
CW1 INPUT
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2011 All rights Reserved
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SoftRock RXTX Ensemble
USB Interface

The trace below shows the keyin on J2-5 and input to the ATTiny85 on PB5.
o Magenta = J2-5
o Green = PB5
o Note R8 adds little or no delay
CW2 INPUT/SDA out
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CFGSR software does not prevent the simultaneous use of PB1 for I2C and
CW2
o In fact this often lead to the USB being “Disconnected” by the PC.
The trace below shows the key being closed during an I2C communications.
o It appears that the I2C may be attempting to drive high the SDA line
instead of letting the pull up resistor bring the line high.
Keying on CW2 interrupting I2C
o Magenta = J2-2
o Green = PB1
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SoftRock RXTX Ensemble
USB Interface

The screen shots below show that the Si570 registers are not readable during
when CW2 is low
o CW2 high
o CW2 low
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2011 All rights Reserved
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SoftRock RXTX Ensemble
USB Interface
PTT ISOLATION
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PTT using PowerSDR-IQ v.1.12.20 Tune button
o Magenta = PB4 after R11 on U4-1
o Green = /PTT on U4-4
o Note time to turn on /PTT is about 6 uSec
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2011 All rights Reserved
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SoftRock RXTX Ensemble
USB Interface
References
1. Robson, Richard R. Sr., WB5RVZ, Ensemble RXTX Project,
http://www.wb5rvz.com/sdr/ensemble/index.htm, online, accessed 2011.
2. Dick Streeland, USBtiny, http://www.xs4all.nl/~dicks/avr/usbtiny/, online,
accessed 2011.
3. UNKNOWN, USB Overview, http://en.wikipedia.org/wiki/USB#USB_1.0,
online, accessed 2011.
4. ATMEL, 8-bit Microcontroller with 2/4/8K Bytes In-System Programmable
Flash,http://www.farnell.com/datasheets/3892.pdf, online, accessed 2011.
5. NXP(Phillips), UM10204 I2C-bus specification and user manual Rev. 03 —
19 June 2007 http://www.nxp.com/documents/user_manual/UM10204.pdf,
p8, online, accessed 2011.
6. LITEON, High Density MountingType Photocoupler LTV-817 series,
http://www.us.liteon.com/downloads/LTV-817-827-847.PDF, p12 -112,
online, accessed 2011.
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AJ4MC
2011 All rights Reserved
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